As a power source of a mobile device, or the like, a lithium ion secondary battery is mainly used. The function of the mobile device or the like is diversified, resulting in increasing in power consumption thereof. Therefore, a lithium ion secondary battery is required to have an increased battery capacity and, simultaneously, to have an enhanced charge/discharge cycle characteristic.
Further, there is an increasing demand for a secondary battery with a high output and a large capacity for electric tools such as an electric drill and a hybrid automobile. In this field, conventionally, a lead secondary battery, a nickel-cadmium secondary battery, and a nickel-hydrogen secondary battery are mainly used. A small and light lithium ion secondary battery with high energy density is highly expected, and there is a demand for a lithium ion secondary battery excellent in large current load characteristics.
In particular, in applications for automobiles, such as battery electric vehicles (BEV) and hybrid electric vehicles (HEV), a long-term cycle characteristic over 10 years and a large current load characteristic for driving a high-power motor are mainly required, and a high volume energy density is also required for extending a driving range (distance), which are severe as compared to mobile applications.
In the lithium ion secondary battery, generally, a lithium salt, such as lithium cobaltate, is used as a positive electrode active material, and a carbonaceous material, such as graphite, is used as a negative electrode active material.
Graphite is classified into natural graphite and artificial graphite.
Among those, natural graphite is available at a low cost. However, as natural graphite has a scale shape, if natural graphite is formed into a paste together with a binder and applied to a current collector, natural graphite is aligned in one direction. When an electrode made of such a material is charged, the electrode expands only in one direction, which degrades the performance of the electrode. Natural graphite, which has been granulated and formed into a spherical shape, is proposed, however, the resulting spherical natural graphite is aligned because of being crushed by pressing in the course of electrode production. Further, the surface of the natural graphite is active, and hence a large amount of gas is generated during initial charging, which decreases an initial efficiency and degrades a cycle characteristic. In order to solve those problems, Japanese Patent publication No. 3534391 (U.S. Pat. No. 6,632,569, Patent Document 1), etc. propose a method involving coating artificial carbon on the surface of the natural graphite processed into a spherical shape.
Regarding artificial graphite, there is exemplified a mesocarbon microsphere-graphitized article described in JP 04-190555 A (Patent Document 2) and the like.
Artificial graphite typified by graphitized articles made of oil, coal pitch, coke and the like is available at a relatively low cost. However, a satisfactory crystalline needle-shaped coke tends to align in a scale shape. In order to solve this problem, the method described in Japanese patent publication No. 3361510 (Patent Document 3) and the like yield results.
Further, negative electrode materials using so-called hard carbon and amorphous carbon described in JP 07-320740 A (U.S. Pat. No. 5,587,255; Patent Document 4) are excellent in a characteristic with respect to a large current and also have a relatively satisfactory cycle characteristic.
JP-A-2003-77534 (Patent Document 5) teaches that excellent high-rate discharge can be achieved by using artificial graphite having highly-developed pores.
WO 2011/049199 (U.S. Pat. No. 8,372,373; Patent Document 6) discloses artificial graphite being excellent in cycle characteristics.
JP-A-2002-270169 (U.S. Pat. No. 7,141,229; Patent Document 7) discloses an artificial graphite negative electrode produced from needle-shaped green coke having anisotropy based on a flow configuration texture.
WO 2003/064560 (U.S. Pat. No. 7,323,120; JP-A-2005-515957; Patent Document 8) discloses an artificial graphite negative electrode produced from cokes coated with petroleum pitch in a liquid phase.